There are many applications in which a flexible high-pressure thermoplastic hose is used for the transmission of volatile or combustible fluids. Since the materials from which hoses are generally manufactured (i.e., rubber or synthetic polymers) are essentially non-conductive, it is a common practice in these applications to incorporate an element of at least limited conductivity within the body of the hose to permit grounding of the hose and the dissipation of static electric charges. A typical application is a paint spray hose in which paints or lacquers are pumped through a flexible hose to a spray gun at relatively high pressures. In some recently developed commercial installations, the pressures may reach as much as 700 kg/cm.sup.2. The flow of fluid materials through the hose may cause static electric charges to build up which, if not discharged to ground, can lead to sparking and explosion of the volatile solvents or other paint vehicles.
To avoid the danger of explosion and fire when inflammable fluids are transmitted through the thermoplastic or rubber innertube of a hose, it is conventional to incorporate a conductive element, herein termed a static drain, within the wall of the hose. These static drains variously have been in the form of sinuous wires, both single strand and braided, helically wound conductive wires, and flexible conductive elements made by interweaving elastic yarns and copper threads. U.S. Pat. No. 3,828,112 discloses the use of an extrusion or tape of semi-conductive synthetic resin and U.S. Pat. No. 4,059,847 discloses the use of a conductive layer of carbon particles laid down within the hose to provide conductivity. In the case of traditional wire reinforced, rubber hose, the wire reinforcement is itself the static drain.
Some of these construction are not entirely satisfactory because the high pressures, repeated pressure pulses or tight bends and flexing which occur in use, may cause the static drain to break from stress fatigue, overstraining and the like. Since the static drain is usually imbedded within the hose, a discontinuity in the static drain is not observable and the resistance of the hose must be periodically checked to be sure the static drain is functional. A less serious problem results from the fact that the free ends of a ruptured wire, if used as the conductive element, may penetrate the cover of the hose and cut the hand of a worker.
It is necessary to ground the static drain to the fitting and this is usually done by use of an intermediate electrical connector. One kind of connector is disclosed in copending application Ser. No. 307,843 of common assignee filed on Oct. 2, 1981. This application discloses the use of a metal ring which is slipped over the nipple of the stem. One side of the ring is seated against a shoulder on the nipple and the other side has a number of upstanding tines (prongs) which embed themselves between the tube and the cover of the hose and engage the static drain. A brad driven into static drain at the end of the hose is another example of an intermediate connector and a further varient is a coating of conductive paint covering the ends of the hose.
These and other devices for grounding the static drain to the fitting are all objectionable because they add to the cost of the hose and they are not completely reliable. Cost is added because of the intermediate connector itself and the manipulative step that must be performed to orient the connector on the hose or the stem. Conductivity may be lost if the connector is not properly assembled and, even if properly assembled, the connector may lose contact with the fitting as by cold flow or creep of the material of the hose.